Senior Software Engineer interview questions

System design

Usually the highest-weighted round at senior level. Requirements first, then a design that fits them.

1. Design a system that ingests a few hundred thousand events per minute and serves aggregates in near real time.

   Strong answers cover: Pinning down freshness and accuracy requirements before choosing components, and being explicit about what you're giving up — exact counts, ordering, or cost.

2. How would you shard a database that has outgrown one primary?

   Strong answers cover: The shard key and why, what queries become expensive afterwards, and the migration path that doesn't require downtime.

3. Two services need to stay consistent. Walk me through your options.

   Strong answers cover: Naming the actual consistency requirement, then comparing approaches — transactional outbox, saga, two-phase commit — on failure behaviour rather than popularity.

Coding

Correctness and communication. Talking through the approach before typing is scored.

1. Given a stream of values, return a running median.

   Strong answers cover: Choosing a structure that matches the access pattern, stating complexity, and handling the even-length and empty cases without prompting.

2. Parse and evaluate a small expression grammar.

   Strong answers cover: A clean separation between tokenising and evaluating, plus explicit decisions about precedence and malformed input.

3. Make this function safe to call from many threads at once.

   Strong answers cover: Identifying the actual shared mutable state, choosing the narrowest synchronisation that works, and saying what you'd measure to know contention is a real problem.

Debugging and operations

How you behave when something is broken and the cause isn't obvious.

1. p99 latency tripled after a deploy but p50 is unchanged. Where do you look?

   Strong answers cover: Reasoning about what affects the tail specifically — lock contention, GC, a cold cache, a slow dependency on a subset of paths — and how you'd confirm rather than guess.

2. Tell me about the worst production incident you were part of.

   Strong answers cover: Your specific actions, what the actual root cause turned out to be versus your first theory, and a follow-up that made recurrence less likely.

3. How do you roll out a change you're not fully confident in?

   Strong answers cover: A flag or staged rollout, the signal that would trigger a rollback, and who is watching it.

Collaboration and scope

Senior titles are given for raising the output of people around you.

1. Describe a technical decision you lost.

   Strong answers cover: A fair statement of the other position, how you committed afterwards, and whether time proved either of you right.

2. How do you review a large pull request from someone more junior?

   Strong answers cover: Separating blocking issues from preferences, and the fact that some feedback belongs in a conversation rather than in line comments.

3. You inherit a service with no tests and frequent incidents. What are your first 30 days?

   Strong answers cover: Stabilise before refactoring — characterisation tests, observability, the highest-frequency failure first — and how you'd get buy-in for the time.

Senior Software Engineer interview FAQ

Should I ask clarifying questions in a system design round?

Yes, and it's scored. Ten minutes of requirements and constraints before drawing anything is normal at senior level; jumping straight to a diagram reads as inexperience.

What if I can't finish the coding problem?

A clearly-reasoned partial solution with correct complexity analysis usually outscores a rushed complete one with bugs you didn't notice. Say what you'd do with more time.

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